Expression of the ggpS gene, involved in osmolyte synthesis in the marine cyanobacterium Synechococcus sp. Strain PCC 7002, revealed regulatory differences between this strain and the freshwater strain Synechocystis sp. Strain PCC 6803.

Synthesis of the osmolyte glucosylglycerol (GG) in the marine cyanobacterium Synechococcus sp. strain PCC 7002 was characterized. The ggpS gene, which encodes the key enzyme (GG-phosphate synthase [GgpS]) in GG biosynthesis, was cloned by using PCR. A 2,030-bp DNA sequence which contained one open reading frame (ORF) was obtained. The protein deduced from this ORF exhibited 85% similarity to the GgpS of the freshwater cyanobacterium Synechocystis sp. strain PCC 6803. The function of the protein was confirmed by generating a ggpS null mutant, which was not able to synthesize GG and thus exhibited a salt-sensitive phenotype. Expression of the ggpS gene was analyzed in salt-shocked cells by performing Northern blot and immunoblot experiments. While almost no expression was detected in cells grown in low-salt medium, immediately after a salt shock the amounts of ggpS mRNA and GgpS protein increased up to 100-fold. The finding that salt-induced expression occurred was confirmed by measuring enzyme activities, which were negligible in control cells but clearly higher in salt-treated Synechococcus sp. cells. The salt-induced increase in GgpS activity could be inhibited by adding chloramphenicol, while in protein extracts of the freshwater cyanobacterium Synechocystis sp. strain PCC 6803 a constitutive, high level of enzyme activity that was not affected by chloramphenicol was found. A comparison of GG accumulation in the two cyanobacteria revealed that in the marine strain osmolyte synthesis seemed to be regulated mainly by transcriptional control, whereas in the freshwater strain control seemed to be predominantly posttranslational.